Sucrose import from photosynthetic tissues into the phloem is mediated by transporters from the low-affinity sucrose transporter family (SUC/SUT family). Furthermore, sucrose redistribution to other tissues is driven by phloem sap movement, the product of high turgor pressure created by this import activity. Additionally, sink organs such as fruits, cereals and seeds that accumulate high concentrations of sugar also depend on this active transport of sucrose. Here we present the structure of the sucrose–proton symporter, Arabidopsis thaliana SUC1, in an outward open conformation at 2.7 Å resolution, together with molecular dynamics simulations and biochemical characterization. We identify the key acidic residue required for proton-driven sucrose uptake and describe how protonation and sucrose binding are strongly coupled. Sucrose binding is a two-step process, with initial recognition mediated by the glucosyl moiety binding directly to the key acidic residue in a stringent pH-dependent manner. Our results explain how low-affinity sucrose transport is achieved in plants, and pinpoint a range of SUC binders that help define selectivity. Our data demonstrate a new mode for proton-driven symport with links to cation-driven symport and provide a broad model for general low-affinity transport in highly enriched substrate environments.

蔗糖从光合组织输入韧皮部是由低亲和力蔗糖转运蛋白家族（SUC/SUT 家族）的转运蛋白介导的。此外，蔗糖重新分配到其他组织是由韧皮部汁液运动驱动的，韧皮部汁液运动是这种输入活动产生的高膨胀压力的产物。此外，积累高浓度糖的水果、谷物和种子等库器官也依赖于蔗糖的这种主动运输。在这里，我们展示了蔗糖-质子同向转运蛋白拟南芥SUC1 的结构，其具有 2.7 Å 分辨率的向外开放构象，以及分子动力学模拟和生化表征。我们确定了质子驱动的蔗糖吸收所需的关键酸性残基，并描述了质子化和蔗糖结合如何强耦合。蔗糖结合是一个两步过程，初始识别是由葡萄糖基部分以严格的 pH 依赖性方式直接结合到关键酸性残基介导的。我们的结果解释了植物中如何实现低亲和力蔗糖运输，并确定了一系列有助于定义选择性的 SUC 结合剂。我们的数据展示了质子驱动的同向传输的新模式与阳离子驱动的同向传输的链接，并为高度富集的底物环境中的一般低亲和力传输提供了广泛的模型。